The increasing application of high strength-to-weight-ratio materials is one of several strategies adopted in pursuit of increased automobile fuel economy, and current automobiles incorporate a wide variety of such materials including high strength steel, high performance aluminum alloys and magnesium alloys. Among this variety of higher strength-to-weight-ratio materials, magnesium and its alloys are attractive, due to the low density of magnesium coupled with their ability to achieve acceptably high strength when suitably processed.
Magnesium is the most chemically active of the commonly-used automotive structural metals and, unless protected, will tend to corrode when exposed to aqueous solutions, particularly when the aqueous solutions are in contact with other metals or contain metallic ions. Treatments and processes to minimize these corrosion tendencies of automotive magnesium alloys have been developed, but in the attachment of magnesium alloys to dissimilar metals, a galvanic cell may be established in the presence of an aqueous electrolyte. In such a cell, the magnesium component will be anodic and preferentially corrode.
Such a cell may be established when mechanical fasteners such as rivets, bolts or screws are used. These fasteners are almost universally fabricated from another metal or alloy—most commonly steel or, less frequently, aluminum—and are in intimate contact with the magnesium. Hence, any attachment of, or to, a thin wall or sheet magnesium component by mechanical fasteners which penetrate into or through the magnesium article, may create a path for ingress of electrolyte and foster localized corrosion in the vicinity of the fastener. Such corrosion may occur even if the magnesium article is coated with a barrier coating to inhibit overall corrosion, or even if one magnesium alloy article is joined to another magnesium article.
Thus there is a need for improved methods and fasteners for joining magnesium alloy articles, particularly thin or sheet articles, to other sheet materials, including other magnesium-based alloys.